The present invention relates to dental prosthetics and more particularly to an improved support post for securing a dental prosthesis, such as an artificial crown, to the root of a tooth. The invention further relates to a method and a device for strengthening a tooth and root from which the nerve has been removed and specifically to a dental post which provides centering for the forming of a bracing filler to internally reinforce the walls of pulpless roots while providing for the support of an artificial prosthesis.
This invention further provides for a secondary reinforcing means surrounding the central dental post and also being capable of following the contours of the tooth root. It is especially significant in those cases where the tooth root is deeply curved and thus requires reinforcing devices that are highly flexible. This secondary strengthening and reinforcing element is a flexibly coiled wire of a generally helicoid shape, which also serves as a matrix for the luting composite filler.
There are a great many shapes used for support posts by the dental art. Similarly, dental posts have been made from a variety of materials including, for example, stainless steel, rigid synthetic polymers, flexible synthetic polymers, opaque materials or light transmitting, translucent or transparent materials.
Previously the configurations of the support posts have varied from a substantially cylindrical shape to having a polygonal cross-section, from having substantially parallel sides, to devices which taper towards the end intended to be most deeply imbedded in the root of the tooth. Support posts have also been formed having somewhat irregular cross-sectional shapes and some of which have a longitudinally centrally divergent shape, such as in U.S. Pat. No. 4,726,770 to Kurer.
The present invention is intended to provide a support post and tooth root reinforcing anchor which insures a secure attachment of a prosthetic coronal replacement, while strengthening the root so as to prevent splitting or cracking of the remaining tooth wall material, while further reinforcing the luting composition retaining the post in place.
According to the present invention, there is provided a tooth reinforcing and prosthetic support anchor post having a central shaft portion, which has a centrally convergent profile, i.e., along its longitudinal axis, and end portions preferably having substantially parallel sides or, alternatively, at least one reverse tapering deep end, intended to be inserted furthest into the root canal of the tooth. The circumferential surface of the centrally convergent shaft portion of the post of this invention is formed in a substantially continuous curve, i.e., a curve without sharp discontinuities, extending in a longitudinal direction parallel to the longitudinal axis of the shaft, and tapers to a central diameter at least 10% narrower than the maximum diameter at each end of the shaft portion.
In a preferred embodiment, the centrally convergent shaft portion of the dental post of this invention is at least 11 millimeters in length, and preferably not greater than 18 millimeters in length; most preferably, the shaft portion has a length of between about 14 and 17 millimeters. The largest diameter at each end of the centrally convergent shaft portion is at least about 0.04 inch in diameter and preferably not more than about 0.07 inch in diameter. The narrowest central section of the post of this invention has a diameter preferably of not less than about 0.02 inch and preferably at least about 0.035 inch, and preferably not greater than about 0.06 inch.
In a most preferred embodiment, the centrally convergent portion is formed integrally with, or intimately connected to, substantially parallel-sided end portions, which each preferably extend at least another 2 millimeters along the longitudinal axis of the post. This permits the user to tailor the overall length of the post, to meet the needs of a particular tooth, without harming the symmetry of the continuously curved centrally convergent shaft portion. Alternatively, at least one of the ends, generally the one extending deepest into the root of the tooth, may be inwardly tapered if that is desired for a particular canal situation. Contrariwise, the end portion intended to directly support the artificial crown, i.e., the end extending outwardly beyond the tooth, may be circumferentially enlarged, relative to the maximum central shaft diameter, if needed to better support the prosthetic crown.
The post can have a substantially circular cross-section, albeit one with a diameter which varies along longitudinal axis of the shaft portion. A shaft portion having a polygonal cross-section, is also within the scope of this invention, if in a particular situation, e.g., it is desired to prevent rotation. Similarly, an oval design will also prevent rotation and avoid the sharp, stress magnifying corners of a polygonal design.
Each end portion can also have a circular cross-section, however, the end portion extending outside of and beyond the tooth canal can most desirably have a polygonal cross-section in order to prevent rotation of the crown and insure a more stable seat for the crown. The precise shape, size and length of the outer end portions, beyond the continuous curve of the centrally converging shaft portion, can be of a variety of shapes or sizes, without affecting the critical shape of the central shaft portion. The end portions can be cut back or otherwise modified, without changing the centrally convergent shaft portion and thus enable ready tailoring for a particular design of e.g., crown or internal root canal.
The dental post of this invention is preferably formed of a rigid material of the type suitable for use in dentistry. In a most preferred embodiment, the material transmits light energy, particularly in the visible and towards the blue end of the spectrum, and includes materials such as a physiologically inert, transparent or optically conductive material, such as a polycarbonate (e.g. Lexan) or other F.D.A.-approved material having the required physical properties for use as a dental support structure. Such a material can provide the needed support as well as permit the transmission of light energy to assist in the curing of the cement material used to hold the post in place within the root canal. Most preferably, the post is also transparent to X-rays, which would permit an unobstructed view of the canal for inspection by the dentist without requiring opening of the canal and removal of the prosthesis and support post. The polymeric transparent material can also be reinforced to increase its strength, as for example by the inclusion of glass or even carbon fibers. In addition to a reinforced polymer resin, a translucent or transparent vitreous material can be used in forming the centrally convergent post. Generally, such vitreous material will be more rigid than even a reinforced polymer resin.
Another aspect of this invention, which may be combined with the centrally convergent dental post, or with dental posts of substantially any other useful shape, is a helicoid flexible wire formed so as to surround the dental post and be as flexible as the dental post if required for steeply curved tooth roots. When the wire and post combination is inserted into the root canal (after the canal has been prepared by generally known dental procedures, such as with a dental reamer),the wire is immersed in the composite luting material used in dentistry, which also holds the post in place; the wire provides a reinforcing matrix for the luting composite, further strengthening the endodontically compromised tooth. The wire forming the coil can have substantially any cross-section. Although generally it is circular in cross-section, the precise shape depends on the method of manufacturing and, for example, if extruded, the cross-section is determine by the shape of the die opening.
The coiled wire can be made of a metallic composition or composites such as reinforced fiberglass rod material or high-density polyesters. The helicoid wire can be utilized together with any type of luting composite, e.g., both self-curing and light curing, known to the dental arts, and which is not antagonistic to the material of the wire or of the dental post. The luting material serves to secure the helicoid wire and the dental post within the root canal walls. The strengthened tooth is better able to withstand the normal forces of normal masticatory function. When the post is formed of a relatively soft resin composition, such as a polycarbonate, or other polymer commonly used in the dental arts, even when it is integrally encased within the luting composite, it can be relatively easily drilled out, to reopen the canal to further work if it becomes necessary; the polymeric post is far more readily drilled out than the composite.
When a steeply curved root is to be strengthened, the dental post and the wire must be extremely flexible. Forming flexible dental posts is well known to the art. Similarly, forming flexible coiled wire is well-known, in the wire extrusion art. Moreover, it must be noted that the flexibility of the coiled wire is substantially inherent as a result of its geometry, even when extremely hard, normally rigid metals are used to form the wires. Generally, dental quality stainless steel or titanium metals and alloys can be utilized for the helicoid wire. The wire is preferably not more than about 0.004 inch in diameter and preferably in the range of from about 0.002 to about 0.003 inch in diameter.
The diameter of the helicoid coil is limited by the diameter of a post that would fit the prepared root canal, and is generally in the range of from about 0.04 inch to about 0.07 inch. The diameter of the coil should be sufficient to surround the dental post and to fit within the prepared canal. The coil need not be a snug fit around the post; indeed, a slack fit, or one which permits some movement of the post relative to the coil, may be preferred in order to encapsulate the wire within the luting composite surrounding the post. However, when inserting the coiled wire and post into the root canal, it is preferred that there be a snug fit so that they both pass into the tooth canal together. Thus, it would be most preferred if means were provided tightly securing the coiled wire to the dental post when it is inserted, and then to permit the helicoid wire to expand within the canal to a position intermediate the outer circumference of the dental post and the inner surface of the canal walls.
One procedure would comprise mounting a coil on a dental post, e.g. one of light-transmitting flexible polymer,and inserting the assembled post and wire coil into a canal filled with luting composite. When multiple canals in a multi-rooted molar tooth are thus treated, each root canal can be filled with luting composite and a coil and post can be inserted into each filled root, to form a solid interconnected structure, strengthened to support the coronal replacement. Furthermore, if the post is removable, as with a relatively soft plastic, it can be removed with conventional dental instruments, if retreatment is subsequently required.
The coiled wire of this invention is not limited to use with the centrally convergent dental post of this invention. It can be used with both light transmitting posts, which would be especially useful when working with light-curing luting composites, or with opaque posts, e.g., metallic dental posts, which generally would be used with self-curing luting composites. Metallic posts are especially useful when threaded, especially when the post thread can mate with the coil helix to permit firmly fastening devices to the root, by providing additional traction, or to permit tightening. When both the coil and the threaded post are of metal, the interface will permit threading, removal and rethreading back into the tooth, without quickly wearing out the threads, as happens with threads formed into the luting composite.
Moreover, in some circumstances, the helicoid wire itself can be utilized to transmit light into the medium, e.g., when it is formed of suitable polymeric material; however, such light-transmitting coiled wire materials, from a structural point of view, would not be as desirable as the stronger but non-light-transmitting metals or reinforced polymers, such as fiberglass or carbon fiber wire.
The coiled wire, when formed of a metal, is likely to form a shadow and thus somewhat discolor the appearance of the tooth. This effect can be overcome, or at least reduced, by the well-known means of coating the coiled wire with a thin layer of an opaque material, such as is often used for metal dental posts, and are themselves well known to the art. Such opaque coatings can be formed from colored pigments mixed with a quick-drying alcohol diluent, as is well known in the dental arts.
Further details of the present invention are shown in the accompanying drawings, by way of example and not by way of exclusion. Many portions of the invention, or the context therefore, are shown in schematic representation, where greater detail is unnecessary as the detail will be apparent or otherwise known to those skilled in the art.